That doesn't mean they're less perilous. We've created the ultimate guide to protecting yourself while pursuing the most passionate forbidden affairs.

A Friend's Ex Resist the urge. Date these women at your own risk.

Number who have indulged: 25 percent of men, 23 percent of women

Why you want her: Your buddy has extolled her virtues so relentlessly, she's become the woman of your dreams, too. "He's basically given you a sales pitch for his girlfriend," says Amy Bippus, Ph.D., a professor of communication studies at California State University at Long Beach.

What to consider: If the separation wasn't mutual, beware. "There's no bigger blow to a man's self-esteem than to be rejected in favor of a friend," says Judy Kuriansky, Ph.D., author of The Complete Idiot's Guide to Dating.

What to Do: If you must act, stay away until he has moved on to another love interest. "There's a psychological threshold that's crossed when a person takes up a new relationship," says Kerner. "Plus, you don't want to look like a scavenger."

A Friend's Sibling Proceed with caution. Tread carefully while pursuing these women.

Number who have indulged: 20 percent of men, 24 percent of women

Why you want her: She has your best friend's personality and Scarlett Johansson's curves, and she already knows your name. "You've crossed the toughest boundary in dating," says Logan Levkoff, a sex columnist.

What to consider: If the relationship sours, don't expect your friend to back you up. "Chances are, he'll protect his sister," says Levkoff. You'll also lose a sounding board for relationship troubles.

What to Do: Say this: "How would you feel if I said I was interested in your sister?" not "Dude, Susie is smoking hot. Can I date her?" "Ultimately, you want to give your buddy a chance to express himself, not deliver a thumbs-up or thumbs-down vote," says Kuriansky.

A Close Friend Make your move. These relationships boast a strong base.

Number who have indulged: 61 percent of men and women

Why you want her: "She has the potential to be everything you need: the friend, the playmate, and the lover," says Levkoff.

What to consider: If you've been a confidant for her most lurid dating stories, chances are she's not open to an upgrade. But if, after hanging out, she starts referring to the two of you as "us," you're in.

What to Do: Choose your words wisely. "I've always loved you" could cheapen the friendship. "Women often feel betrayed if they find out you've been sexually interested all along," says Kerner. "Tell her she's great and ratchet up the intimacy a bit. It's better to let it happen naturally than to deliver a stump speech."

A Coworker Make your move. These relationships boast a strong base.

Number who have indulged: 35 percent of men and women

Why you want her: "The workplace is a trove of attractive, like-minded people our own age who dress nicely," says Bippus. "It's deep intimacy with rigid barriers," adds Helen Fisher, Ph.D., author of Why We Love. "That's a recipe for romantic love."

What to consider: Nothing kills a meeting—and your reputation—faster than puppy-dog eyes across a conference table. "Couples always think other people don't know, but they always do," says Marie McIntyre, Ph.D., author of Secrets to Winning at Office Politics. The obvious pitfall: You split and loathe each other, all under the eyes of the office gossips.

What to Do: You can have an office romance, as long as you keep the office and the romance separate. "You need to prove to your boss that your relationship status won't change the work environment," says McIntyre. A reassuring heads-up to your supervisor once you're exclusive could help reinforce your professional image. If a breakup comes, go ahead and have the screaming fight, but then settle how you'll deal in the office. Your jobs depend on it.

An Intern Proceed with caution. Tread carefully while pursuing these women.

Number who have indulged: 18 percent of men, 9 percent of women

Why you want her: She's young, hot, well dressed, helpful, and somehow convinced that you're a lot better at your job than you actually are.

What to consider: Learn from Lewinsky: Nothing grabs attention—at the watercooler or the newsstand—like intern gossip. What's worse, she's a walking lawsuit. "Friendly advances can be mistaken as sexual harassment," warns McIntyre.

What to Do: Hold your horses. Even if she's interested. Even if you bump into her outside of work. Even if she's the one slurring come-ons at happy hour. "As soon as she's packed up her cubicle, you've got the green light," says McIntyre. Until then, let the tension build to a passion-packed payoff.

Your Boss Resist the urge. Date these women at your own risk.

Number who have indulged: 8 percent of men, 7 percent of women

Why you want her: She's great at telling you what to do. How might that boardroom attitude transfer to the bedroom? Another aphrodisiac: This relationship requires secrecy. That's sexy. At least until it becomes annoying.

What to consider: Remember how that one ex-girlfriend set your clothes on fire? Now imagine dumping your boss. Also, once coworkers get wind of the situation, they'll assume you're receiving unfair attention. Which you are.

What to Do: Nothing. Simmering tension can work in your favor forever, if you leave it at that. If a relationship takes hold, you have another decision to make: Is the job or the relationship more replaceable? One way it can work: in sprawling workplaces with several supervisors, says Robyn DeVal, Ph.D., a marriage-and-family therapist. "There's less opportunity for overlap between your personal and professional lives."

A Sister's Friend Make your move. These relationships boast a strong base.

Number who have indulged: 13 percent of men, 7 percent of women

Why you want her: She's the safe choice. "Meeting people is hard, and dating someone you know is easier and safer," says Norah Dunbar, Ph.D., a specialist in interpersonal communications at California State University at Long Beach.

What to consider: "Some siblings aren't comfortable merging those worlds," says Kerner. "You run the risk of harming her relationship with a friend."

What to Do: You don't need your sister's blessing, but she could help you seal the deal. "Most siblings are willing to play matchmaker," says Kerner.

An Ex's Friend Resist the urge. Date these women at your own risk.

Number who have indulged: 20 percent of men, 37 percent of women

Why you want her: She's the Long Island iced tea of dating: a bunch of bad stuff mixed together that makes you feel good until the brutal hangover sets in. Even worse: You've just upped your creep quotient exponentially.

What to consider: "Women don't handle this situation as well as men do," says Eve Marx, author of Flirtspeak: The Sexy Language of Flirting. "You have a 99 percent chance of losing her as a friend."

What to Do: Rein in your impulses. But if she's the predator, the burden is on her to bridge the gap with your ex.